1. Field of the Invention
The present invention relates generally to a mid-pressure cleaning system that uses dense-phase fluids to remove photoresist and photoresist processing residues. In particular, the present invention relates to a system and method for mid-pressure dense phase gas and ultrasonic cleaning. By mid-pressure it is meant that the cleaning system is generally used at pressures between atmospheric and about 1500 psi.
2. Description of the Related Art
Generally, semiconductor substrates, such as circular wafers, are processed into semiconductor chips by sequentially exposing each substrate to a number of individual processes, such as photomasking, etching, implantation, and cleaning. Cleaning typically consists of removing resist and/or etching residue from the surface of the substrate.
Generally, there are two methods for cleaning the surface of a substrate, namely, wet and dry processing. Wet processing consists of a series of steps of spraying and/or immersing the substrate in chemical solutions or solvents. These conventional solvent-aided cleaning processes are currently being re-evaluated due to environmental concerns. In addition, recent environmental legislation mandates that many of the organic solvents used in wet processes be banned or their use severely limited. Dry processing, on the other hand, consists of a series of steps that use gases instead of wet chemical solutions to clean the substrate. For example, ashing using an O2 plasma. However, such processes often leave a residue after dry cleaning, which is unacceptable because such residue may cause device failures or limit operation efficiency.
More recently, dense phases gases or fluids, such as carbon dioxide (CO2) with or without co-solvents or surfactants, have been introduced to remove etch residue and/or photoresist from semiconductor substrates. A dense phase fluid is a gas compressed to either supercritical or subcritical conditions to achieve liquid-like densities. These dense phase fluids or fluid mixtures are also referred to as dense fluids. Unlike organic solvents, such as n-hexane or 1,1,1-trichloroethane, dense fluids exhibit unique physical and chemical properties such as low surface tension, low viscosity, and variable solute carrying capacity.
Cleaning with dense phase fluids is desirable, as such fluids retain the properties of a liquid, but have the diffusivity and viscosity of a gas. In addition, dense phase fluid cleaning technology can be applied in many industrial processes to significantly reduce or eliminate the use of hazardous chemicals, to conserve natural resources such as water, and to accomplish tasks previously not possible, such as rapid precision cleaning of small features of semiconductor devices (e.g., resist images, VLSI (Very Large Scale Integration) topographical features such as vias, etc.). Dense-phase fluids act as a solvent to remove contaminants from the wafer surface and effectively clean the surface of the substrate. Additionally, as required, the cleaning effectiveness of dense phase fluids is typically enhanced by the addition of chemical agents or co-solvents that react with materials used in semiconductor manufacturing.
However, typical modern dense-phase fluid cleaning systems and methods use high pressures, sometimes on the order of 4,000 psi or more, to generate and maintain dense-phase fluids used for cleaning. These high pressures typically require complicated and expensive pumping systems, as well as condenser systems for creating and maintaining the dense-phase state. Furthermore, these systems become more expensive as the process is scaled up, due to greater engineering and construction demands placed on the pumping systems and pressure vessels, as the size of the associated process machinery increases. Even further, these systems are inflexible because they require that all components to be cleaned are exposed to dense-phase fluids under the same pressure and temperature conditions, regardless of the individual cleaning requirements for each component. This often results in needless additional processing time and expense.
Therefore, what is needed is an inexpensive alternative to current dense-phase fluid cleaning systems.